This invention relates to the field of DC/DC converters and, more particularly, to a converter which can operate at maximum efficiency at two different output current demand levels.
DC/DC converters are widely used in systems requiring voltages higher than the power supply or battery can provide, and it is well known that a semiconductor switch responding to a pulse width modulated input signal can be used to supply current to an inductor/diode/filter combination to generate a DC voltage which is higher than the B+ supply. Such circuits usually are designed for maximum efficiency at the maximum expected current output, thus the power used to drive the switch is optimized for that maximum current. If the required output current is less than that maximum, drive power is wasted which results in very inefficient operation and the possible degradation of the converter. In some known converters, the excess drive current is "dumped" into a "sink" circuit which improves the quality of the operation, but this approach is wasteful of power.
Also, prior art converters have typically required B+ supplies of a much higher voltage than is typically available in personal electronic products such as pocket pagers. The presently known DC to DC conversion techniques are not well suited for use in portable battery operated equipment, such as paging receivers, where the power source is a small, low voltage battery. In such applications, it is highly desirable to minimize the power drain from the battery to provide acceptable and economical battery life while keeping the physical size of the battery small.
Thus, the prior art approaches are particularly ill-suited for applications where the output load of the converter is switched between two drastically different demand levels, as for example, is the case when a CMOS microcomputer paging decoder is cycled between a high drain, fully operational mode and a reduced drain standby mode. Here, the power saving benefits of the switched mode operation of the microcomputer can only be fully realized if the DC/DC converter operates with high efficiency under the two different load conditions.